The present invention relates to linear motors and, more particularly, to apparatus for sensing and controlling the position of a movable element of a linear motor.
Electric motors conventionally include a stationary portion, or stator, and a movable portion. In electric motors adapted for turning an output shaft, the movable portion, called the rotor, is concentrically arranged with the stator. Although the most common rotary motor employs a cylindrical rotor rotating within an annular stator, the reverse arrangement is not unknown.
An electric motor of the linear type has more recently found use in certain applications. A linear motor employs a stator which can be thought of as being similar to the stator of a rotary motor which has been developed, or opened out, into a flattened assembly. Similarly, the rotor of the rotary motor is replaced by a linear counterpart which moves in a line with respect to the stator.
Alternating current versions of linear motors of the synchronous type are disclosed in U.S. Pats. Nos. 3,594,622; 3,699,365 and 3,706,922, among many others. Linear motors of the induction type are disclosed in U.S. Pats. Nos. 3,770,995; 3,824,414 and 3,884,154, among many others.
An even more recent type of linear motor employs direct current using either wound field-generating coils on both the moving and stationary elements or using one wound element and one element which employs one or more permanent magnets. Such a linear motor is disclosed in U.S. patent application Ser. No. 383,351, now Pat. No. 4,560,911 invented by the present applicant. In the referenced patent application, a stator employs a U-shaped channel to contain the stator field-generating elements. A movable element is guided to move along the axis of the U-shaped channel. An apparatus to be linearly displaced is connected to the moving element.
In some cases, the stator of a linear motor can be very much longer than its moving element. For example, a linear motor stator measuring several feet long may be employed with a movable element only a few inches long. When the stator contains wound coils, it is only those coils in the vincinity of the moving element which are effective to interact with the magnetic field of the moving element to produce a motive force. All of the coils which are a substantial distance away from the moving element make no contribution to the work of the motor. Thus, in order to obtain improved motor efficiency, a linear slip ring technique is disclosed in the referenced patent application to apply power only to those stator coils facing, or immediately adjacent to, the movable element. Part of the linear slip rings perform the functions of a conventional commutator.
In their commutator functions, the slip rings perform the following:
1. they select, and apply power to, those stator coils in the vicinity of the moving element and PA1 2. they reverse the polarity of the power applied to the coils at the proper points in the travel of the moving element as a function of the relationship between the positions of the magnetic poles on the stator and the moving element.
The latter function produces essentially instantaneous voltage reversal of the power fed to the energized coils. In some circumstances, this can result in force pulsations applied to the movable element.
Modern high-power linear motors of the type disclosed in the referenced patent application employ rare-earth permanent magnets having a very high field strength exerted over a very small gap. In some cases, the attractive force between the permanent magnets and the magnetic material in the stator can reach a value of several hundred pounds. This can increase the support structure required to maintain the movable element in position and to permit it to move along its axis.